Batteries have become increasingly important in modern society, both in powering a multitude of portable electronic devices, as well as being key components in new green technologies. These new technologies offer the promise of removing the dependence on current energy sources such as coal, petroleum products, and natural gas which contribute to the production of by-product green-house gases. Furthermore, the ability to store energy in both stationary and mobile applications is critical to the success of new energy sources, and is likely to sharply increase the demand for all sizes of advanced batteries. Especially for batteries for large applications, a low base cost of the battery will be key to the introduction and overall success of these applications.
Conventional batteries have limitations, however. For example, lithium ion and other batteries generally employ a liquid electrolyte which is hazardous to humans and to the environment and which can be subject to fire or explosion. Liquid electrolyte batteries are hermetically sealed in a steel or other strong packaging material which adds to the weight and bulk of the packaged battery. Conventional liquid electrolyte suffers from the build-up of a solid interface layer at the electrode/electrolyte interface which causes eventual failure of the battery. Conventional lithium ion batteries also exhibit slow charge times and suffer from a limited number of recharges since the chemical reaction within the battery reaches completion and limits the rechargeability because of corrosion and dendrite formation. The liquid electrolyte also limits the maximum energy density which starts to break down at about 4.2 volts while often 4.8 volts and higher are required in the new industry applications. Conventional lithium ion batteries require a liquid electrolyte separator to allow ion flow but block electron flow, a vent to relieve pressure in the housing, and in addition, safety circuitry to minimize potentially dangerous over-currents and over-temperatures.
With respect to alkaline batteries which rely on the transport of OH− ions to conduct electricity, the electrolyte becomes saturated with ions (e.g., zincate ions during discharge of Zn/MnO2 batteries) at a certain point and eventually the anode becomes depleted of water. In rechargeable alkaline batteries, the reactions are reversed during charge. Formation of the same ions which saturated the electrolyte may hinder discharging, however. The cathode reaction results in the release of the OH− ions. The formation of soluble low valent species (e.g., Mn species during discharge of Zn/MnO2 batteries) can adversely affect the utilization of active material however. Although MnO2 can theoretically experience 2-electron reduction with a theoretical capacity of 616 mAh/g, in practice a specific capacity close to theoretical 2-electron discharge has not been demonstrated. Crystalline structure rearrangement with formation of inactive phases and out-diffusion of soluble products limits cathode capacity.
U.S. Pat. No. 7,972,726 describes the use of pentavalent bismuth metal oxides to enhance overall discharge performance of alkaline cells. Cathode containing 10% AgBiO3 and 90% electrolytic MnO2 was shown to deliver 351 mAh/g to 0.8V cut-off at 10 mA/g discharge rate, compared to 287 mAh/g for 100% MnO2 and 200 mAh/g for 100% AgBiO3. The 351 mAh/g specific capacity corresponds to 1.13 electron discharge of MnO2 and represents the highest specific capacity delivered at practically useful discharge rates and voltage range. Bismuth- or lead-modified MnO2 materials, disclosed in U.S. Pat. Nos. 5,156,934 and 5,660,953, were claimed to be capable of delivering about 80% of the theoretical 2-electron discharge capacity for many cycles. It was theorized in literature [Y. F. Yao, N. Gupta, H. S. Wroblowa, J. Electroanal. Chem., 223 (1987), 107; H. S. Wroblowa, N. Gupta, J. Electroanal. Chem., 238 (1987) 93; D. Y. Qu, L. Bai, C. G. Castledine, B. E. Conway, J. Electroanal. Chem., 365 (1994), 247] that bismuth or lead cations can stabilize crystalline structure of MnO2 during discharge and/or allow for 2-electron reduction to proceed via heterogeneous mechanism involving soluble Mn2+ species. Containing said Mn2+ species seems to be the key for attaining high MnO2 utilization and reversibility. In high carbon content (30-70%) cathodes per U.S. Pat. Nos. 5,156,934 and 5,660,953, the resulting highly porous structure was able to absorb soluble species. However, there is no data to suggest that a complete cell utilizing these cathodes was built or that this worked using a Zn anode.
Accordingly, polymer electrolyte which prevents 1) the dissolution of ions which would otherwise saturate the electrolyte and 2) the dissolution and transport of low-valent species, would improve utilization and rechargeability of alkaline batteries. In addition, it has been suggested [M. Minakshi, P. Singh, J. Solid State Electrochem, 16 (2012), 1487] that Li insertion can stabilize the MnO2 structure upon reduction and enable recharegeablity. A polymer engineered to conduct Li+ and OH+ ions, opens the possibility to tune MnO2 discharge mechanism in favor of either proton or lithium insertion, which can serve as an additional tool to improve life cycle.
Further, while the battery technology for many advanced applications is Lithium Ion (Li-ion), increased demands for higher energy density, both in terms of volumetric (Wh/L) for portable devices, and gravimetric (Wh/kg) for electric vehicles and other large applications have shown the necessity for accessing technologies well beyond the current capabilities of Li-ion cells. One such promising technology is Li/sulfur batteries. A sulfur based cathode is enticing because of the high theoretical energy density (1672 mAh/g) which is ˜10× better than the current Li-ion metal oxide cathode active materials. Sulfur is also exciting because it is a very abundant, low cost, environmentally friendly material, unlike many current Li-ion battery materials, such as LiCoO2.
Recently, there has been a great amount of activity in Li/sulfur battery research, with advances in the capacity and cycle life of rechargeable Li/sulfur cells. Activity has included modifications to the cathode, anode, electrolyte and separator, all with the goal of reducing the polysulfide shuttle and thereby improving cell performance. Applications of this research to sulfur cathodes has focused in two main areas: 1) the use of engineered materials to surround and contain the sulfur and soluble lithiated products, for example see: U.S. Patent Application 2013/0065128, and 2) the use of conductive polymers which react with sulfur to produce a “sulfurized” composite cathode material. Examples of “sulfurized-polymer” include reaction products from high temperature exposure of sulfur with polyacrylonitrile (PAN) [see: Jeddi, K., et. al. J. Power Sources 2014, 245, 656-662 and Li, L., et. al. J. Power Sources 2014, 252, 107-112]. Other conductive polymer systems used in sulfur cathodes include polyvinylpyrrolidone (PVP) [see: Zheng, G., et. al. Nano Lett. 2013, 13, 1265-1270] and polypyrrole (PPY) [see: Ma, G., et. al. J. Power Sources 2014, 254, 353-359]. While these methods have met with various degrees of success in limiting the polysulfide shuttle mechanism, they all rely on the use of expensive materials which are not well suited to large scale manufacturing.